Method of improving photographic silver halide emulsions



United States Patent 6,005,713.... METHOD OF IMPROVING PHDTOGRAPIHC SILVER HALIDE EMULSIONS Fritz Der-sch, Binghamton, N.Y., assignor to General Aniline & Film Corporation, New York, N.Y., a corporation of Delaware I No Drawing. Filed Aug. 11, 1960, Ser. No. 48,854 5 Claims. (Cl. 96109) This invention relates to photographic silver halide emulsions and particularly to the incorporation of a reaction product of gelatin with N-acetylhomocysteine thiolactone (AHTL) in such emulsion for the purpose of increasing speed or contrast, or both, of such emulsion, while reducing the fog and preventing loss of speed.

Photographic silver halide emulsions are usually prepared by mixing soluble silver salts, e.g., silver nitrate with soluble silver halides, e.g., potassium bromide, potassium iodide or sodium chloride in the presence of colloids such as gelatin, carboxyethyl cellulose, water soluble casein, polyvinyl alcohol or others. The selection of gelatin for thispurpose is very critical and requires that the gelatin be carefully tested. Very often the photographic properties of gelatin containing emulsions, such as speed, contrast, freedom from fog or stability in storage, depend on the presence of minute impurities in gelatin. Some of these substances, called ripeners or sensitizers, exert a ripening or sensitizing efiect, that is, a speed or contrast increasing effect on the silver halide emulsion. Gelatin, being a natural product and derived from various sources such as hides, bones, tendons, sinews and from difierent animals such as calf, cow, steer, pig, whale and being manufactured by different processes, may containmore or less or none of these substances. The nature or chemical structure of these substances is not in all cases completely known, although some authors have postulated that they are sulfur containing compounds. Allylisothiocyanate, allylthiourea, sodium thiosulfate and polythionates have been described in the literature as the possible photographically active compounds present in the gelatin. It has been proposed also by some workers to add these or similar compounds to gelatin, gelatin derivatives or gelatin substitutes. Adding these compounds to gelatins or to photographic emulsions very often leads to other complications, e.g., to theiormation of fog. I V

I have now found that photographic gelatin which has been thiolated by chemical treatment with AHTL and the disulfides resulting from oxidation of such thio lated gelatin possess built-in ripening or sensitizing properties and as such do not depend on the presence of a more or less unknown amount of impurities in the gelatin.

The addition to a photographic silver halide emulsion or to photographic gelatin or gelatin substitute used for the manufacture of such emulsion of a predetermined and controlled quantityvof the AHTL derivative of gelatin or the disulfide resulting therefrom by oxidation and emulsions so produced constitute the purposes and objects of my invention.

Usually the method used for incorporating the aforesaid derivatives of gelatin (thiolated gelatin or the disulfideproduced by its isolation) consists of dissolving the derivative in water and adding this solution to the emulsion at a stage where its efiectiveness is most beneficial. Normally this is at the point where gelatin is added to -a photographic emulsion. An alternative method is to incorporate the derivative in the gelatin or gel substitute employed in preparing the photographic emulsion.

The preparation of the N acetylhomocysteine thiolactone (AHTL) derivative of a gelatin is carried out ac- Patented Oct. 24., 1261 2 cording to the method described by R. Benesch and R. E. Benesch in the National Academy of Sciences of the U.S. Proceeding $44 848-53 (September 1958).

The reaction proceeds according .to the following The reaction is carried out by treating an aqueous solution containing the gelatin-( 3-5 aud AHTL (of. Table I below) with alternate increments of AgNO and NaOH to keep the pH at 7.5 until a total of 1 mol of silver per mol of AHTL has been added. The reaction is complete when all the AG-AHTL complex has dissolved. This takes about one hour. The clear yellow solution of the silver mercaptide of the thiolated protein is adjusted to pH 2.5 and enough thiourea is added to convert all of the silver into the soluble Ag(thiourea) complex. This complex ion is removed with any commercial cation exchange resin and the protein is washed off the resin with acidified l M thiourea solution. The effiuent protein solution is then brought to pH 7 and passed through an anion exchange resin (cf. U.S. Patent 2,591,573) in order to remove some N-acetylhomocysteine which is formed as a byproduct due to hydrolysis of AHTL. The protein solution is finally freed of thiourea and salts by dialysis under nitrogen and lyophilized by removing water by evaporation at extremely low temperature and high vacuum.

TABLE 1 Thiol ation of gelatin Groups per 10 gm.

Moles Reagent per Initial SH -N H2 Group NH Formed 1 Final NH NH: Blocked l Determined both according to P. D. Boyer, I. Am. Chem. 300., 76,

' 4331, 195i, andR. Benesch and R. E. Benesch, Biochem. et. Biophys.

Water Gelatm Soluble 7 S oxidation dilute solution Thiolated Gelati.u+GelatinSHr Kxidation cone. solution GelatinSS-Gelatin The preparation of the water soluble disulfides can interchange with G elatin-SH be carried out according to the methods described by Benesch and Benesch (ibidz);

These authors use potassium terricyanide as the oxidizing agent. A 0.2 percent solution of thiolated gelatin is treated with a slight exces of 0.1 M potassium ferricyanide at pH 7 until the nitroprusside test is negative, Ferricyanide and ferrooyanide ions are removed with an anion exchange resin of U.S. Patent 2,591,573. After dialysis the solution is lyophilized. The product thus obtained is freely soluble in water.

As noted, this water soluble disulfide can also be used within the scope of my invention. It has thedesirable faculty of reducing the fog tendencies during prolonged ripening.

The quantity of the thiolated gelatin or the water soluble disulfide derived therefrom by oxidation is relatively small compared to the quantity of the emulsion or the gelatin or gel substitute employed. Usually amounts of from about .05 to .5 gram of the additive is employed per kilo of emulsion or gelatin or gel substitute. a

Any photographic gelatin may be utilized for my purposes. It is found that by the involved reactions such gelatins are converted to products having the characteristics contemplated herein.

The thiolated gelatin and the water soluble disulfide have another very desirable and entirely unexpected property, namely, they prevent speed recession in storage and during the time prior to coating when the emulsion is melted andheld in a liquid state; i

The invention is further illustrated by the following examples but it is to be understood that it is not limited thereto.

. 1 EXAMPLE I 62.5 grams of silver nitrate were dissolved in 625 cc. of distilled water. 45 grams of potassium bromide and 6.25'grams of gelatin were dissolved in 228 cc. of disstilled water. To this solution were added 21.9 cc. ofa 10% potassium iodide solution. 250 cc. of this silver nitrate solution were added to the bromide-iodide gelatin solution in, one minute and the rest of the silver nitrate solution in 24. minutes. Then 125 grams of gelatin, dissolved in 300 cc. of water were added and the batch digested for a period of 25 minutes. The pH of the emul- Type emulsion as above but not containing the AHTL treated gelatin during the digestion:

The oven test consists of a six-day incubation of the photographic test material at 53% relative humidity at sion at this stage was adjusted to 6.8.. During the mixing 7 and digestion, the temperature was held at 71 C. The emulsion was then cubed, washed and submitted to a second ripening process for which purpose the emulsion was melted in 2.0,minutes and heated to 49 C. ;The digestion was carried out for two hours. At the beginning of the digestion, 6 cc. of a 1% aqueous solution of Grams Metol 1.5 Sodium sulfite, anhydrous Sodium bisulfit 1 Hydroquinone 3 Sodium carbonate, monohydrated 6 Potassium bromide .8

Water to make 1 liter.

Experimental emulsion as previously described:

Fresh Test Oven Test Relative Speed 6 Speed at Fog 6 Dev. Dev. Fog 6' Dev.

6' Dev.

These results establish the absence of speed recession in the emulsion containing AHTL treated gelatin added during the digestion of the emulsion.

EXAMPLE II' In this example, the AHTL treated gelatin was applied as a coating final and not as an additive during the digestion, that is, it was added prior to coating while the emul- 'sion was being kept in the liquid state.

' 1 kilo of a gelatino-silver halide emulsion containing 2% silver iodide and 98% silver bromide was prepared in a conventional manner and brought up to its maximum light sensitivity. It was then readied for coating and finals were added such as sensitizing dyes and hardening agents. Quantities of AHTL treated gelatin, as indicated in Table II, were added to the emulsion in the form of a 1% aqueous solution. The emulsion samples contained about 0.6 mol of silver halide. The soprepared emulsion samples were coated on a suitable cellulose ester base and dried. Samples of these film coatings were then exposed in a Type IB Sensitometer and developed in a developer of the composition of that described in Example I.

TABLE II Relative Fog 6' Fog 12 Quantity of Compound Used Speed 6' Dev. Dev.

Dev.

0 Emulsion held at40 0-30 prior to coating 100 26 40 D Emulsion held at 40 C. 4 hoursprlor to coating 70 26 .40 400 mg. Emulsion held 4 hours 75 24 .37 800 mg. Emulsion held 4 hours 26 39 1.6 g Emulsion held 4 hours '90 28 41 3.0 g. Emulsion held 4 hours 24 35 These results show appreciably reduced speed loss during the holding of the emulsion at 40 C. in the melted state prior to coatingin the cases where the AH'PL treated gelatin was present in the emulsion.

. i EXAMPLE IH Thisexample is similar to Example I with the exception that in place of the AHTL treated gelatin the water soluble oxidation product of such a gelatin derivative was used. 'For reasons of simplicity this compound is -Thi$ examp e shows that the oxidation product of the 5 AHTL treated gelatin is an excellent antiioggant in photographic silver halide emulsions.

Modifications of the invention will occur to persons skilled in the art. I, therefore, do not intend to be limited in the patent granted except as necessitated by the appended claims.

I claim:

1. A photographic silver halide emulsion containing a small amount of a gelatin derivative selected from the class consisting of the reaction product of gelatin with N-acetylhomoc'ysteine thiolactone and the water soluble disulfide oxidation product thereof.

2. A photographic silver halide emulsion containing a small quantity of the reaction product of gelatin with N-acetylhomocysteine thiolactone.

3. The process of reducing the tendencies of a silver halide emulsion to speed recession in storage and prior to coating the emulsion when held in the melted state, which comprises adding to the silver halide emulsion a small quantity of a gelatin derivative selected from the class consisting of the reaction product of gelatin with N-acetylhomocysteine thiolactone and its water soluble disulfide oxidation products.

4. The process as defined in claim 3 wherein the gelatin derivative is added during digestion of the emulsion.

5. The process as defined in claim 3 wherein the gelatin derivative is added just prior to coating and while the emulsion is in the melted state.

No references cited. 

1. A PHOTOGRAPHIC SILVER HALIDE EMULSION CONTAINING A SMALL AMOUNT OF A GELATIN DERIVATIVE SELECTED FROM THE CLASS CONSISTING OF THE REACTION PRODUCT OF GELATIN WITH N-ACETYLHOMOCYSTEINE THIOLACTONE AND THE WATER SOLUBLE DISULFIDE OXIDATION PRODUCT THEREOF. 